A major problem of neuroscience is to understand the mechanisms of behavioral recovery following neural injury. Findings from our laboratories during the present project of non-associative learning by nerve regeneration and suggest new mechanisms of non-associative learning. Thus we propose to use the leech as a model system that study the cellular basis of functional recovery following injury. This entirely collaborative work is the basis for the proposed cellular, and behavioral studies on recovery of the capacity of one form of learning, sensitization, following damage to central circuits. The project represents the convergence of our separate behavioral and synapse regeneration studies in the leech, combining in depth work on modulation of defensive shortening from one laboratory and work on sensory neurons and on the 5 interneuron, its connections and its responses to injury from the other laboratory. The proposed experiments will provide insight into the questions: (1) What cellular changes underlie learning, including non-associative learning, and (2) by what cellular mechanisms does the nervous system recovery from injury? The leech is particularly advantageous for studies of this type because its ganglia contain identifiable neurons capable of regenerating specific connections following axotomy, because individual neurons can be selectively ablated or their axons cut without injury to surrounding structures, and because stereotyped adult behaviors can be examined both in vivo and in vitro while recording from identified neurons, including the S cell. The proposed experiments will determine (1) the mechanism increasing S cell activity during sensitization of reflexive shortening, including the relative roles of serotonin, relief of conduction block and reflection of impulses in mechanosensory neurons; (2) how signals are conveyed from the sensitizing stimulus, especially involving the S cell; (3) the mechanism by which sensitization is restored following repair of the S cell axon and its synapse properties of S cell connections in relation to nitric oxide synthase (NOS) activity. We shall focus on sensitization, which is eliminated by S cell ablation, rather than on dishabituation, which is only impaired. The methodology will include electrophysiological recording, intracellular injection of morphological markers, laser microbeam axotomy, behavioral testing of leeches including semi-intact preparation, immunocytochemistry, and confocal and electron microscopy. The proposed studies will reveal cellular circuitry for non-associative learning and basic mechanisms for restoration of plastic properties of the nervous system following injury.